An MEMS (Micro Electro Mechanical Systems) structure is so far known in which an excitation portion including a piezoelectric thin film is constituted on a Si semiconductor layer. Various trials have been made to improve the temperature coefficient of resonant frequency (TCF) in a vibrator with the MEMS structure. Patent Document 1, given below, discloses a method of reducing an absolute value of the TCF by laminating Si and SiO2. Patent Documents 2 and 3, given below, disclose a method of reducing the primary temperature coefficient of resonant frequency of Si itself by doping a p-type or n-type dopant into Si.
Patent Document 4, given below, discloses a method of using a Si/SiO2 combined material and doping Si at a high concentration. Patent Document 4 states that the secondary temperature coefficient of resonant frequency can be reduced.    Patent Document 1: WO2008/043727    Patent Document 2: WO2010/062847    Patent Document 3: WO2012/110708    Patent Document 4: WO2012/156585